Updated: Mar 9
Stratocumulus clouds are likely the most common cloud on the planet. They occur just about everywhere in the world and are usually quite harmless. But every once in a while, they can get ugly, especially when it comes to airframe icing.
Back in early February, this urgent pilot weather report from a Mooney M20P got my attention. Severe rime ice at at 1,600 feet MSL. The remark in the pilot report indicated 1.5 inches of ice accreted.
Urgent PIREP M20P
Time: Feb 8, 2020 1443Z
Flight level: 16
Icing intensity: Severe
Icing type: Rime
Urgent PIREP: MTO UUA /OV MTO010010/TM 1443/FL016/TP M20P/IC SEV RIM 1 1/2 INCH OF ICE/RM ZKCFDC/AF
It wasn't more than about 20 minutes later that the Aviation Weather Center issued a SIGMET for severe ice below 6,000 feet likely due to this pilot's urgent report.
The visible satellite image shows a widespread area of clouds covering most of east-central Illinois providing for overcast conditions. The tops of the clouds in the region of the pilot report don't have much in the way of texture indicating they are likely capped. But this doesn't say much about the height or depth of the clouds.
Stratocumulus clouds can often develop in the wake of a cold front. In this case, the deck was present along and to the west of a stationary front. Below is the surface analysis valid at 1200Z (the pilot report was at 1443).
The real important information comes from the color-enhanced IR satellite image below. This depicts the temperature of the cloud tops (or surface of the earth in clear skies). Two important things come to mind. First, the area near the pilot report has a consistent temperature implying the clouds are capped by a temperature inversion. Second, the cloud top temperature is -12°C to -15°C based on the yellow to slightly pale green color in the region of the pilot report. This indicates the cloud below is dominated by supercooled liquid water.
A quick peek at the 1 hour forecast sounding for the location helps to paint a better picture. We don't know if the pilot accreted the ice in a climb or descent through the cloud deck even though the pilot report was for 1,600 feet MSL. This temperature profile is very characteristic of a statocumulus deck given the moist instability in the clouds (saturated conditions follow the moist adiabatic lapse rate) and there's an isothermal layer (temperature remains constant with height) at the tops that are depicted around 5,000 feet with a cloud top temperature of -11° to -12°C. While not exactly the typical healthy inversion that you see with stratocumulus decks, it's still substantial enough to stop or limit the vertical growth of the clouds.
This kind of profile is synonymous with moderate to severe icing. Although we find the drop sizes in these clouds tend to be smaller creating a small drop icing environment and within the envelope of an aircraft with a certified ice protection system (IPS). However, when there's snow cover on the ground as there was in this case, the clouds tend to be clean (less ice and condensation nuclei) and the drop sizes tend to be larger creating a runback hazard. That may not have been a significant factor in this case, but given the severe icing reported by the pilot and the SIGMET for severe ice, the liquid water content was likely higher than usual, possibly enhanced by the proximity to the stationary front.
Most pilots are weatherwise, but some are otherwise™
Weather Systems Engineer
CFI & former NWS meteorologist